Apoptosis (a Greek word, meaning ‘falling off’) is a crucial event during aging. It is the orderly death of cells which are no longer functional or which are beyond repair. Apoptosis (also called ‘programmed cell death’) was first described in 1972 by Kerr, Wyllie and Currie (1) and it has been attracting substantial research interest since. The reason is that a better understanding of apoptosis will help explain some age-related changes AND suggest directions for cancer treatments.
During apoptosis cells die following a set pattern and are methodically eliminated by the organism. Apoptosis is different from necrosis (when cells die at random). In necrosis, healthy and unhealthy cells may die indiscriminately and the process generates toxic material which kills other cells down the line. Instead, in apoptosis, cells are eliminated in a controlled manner which minimizes the damage to other cells.
It is important to have at least a general idea of the complexity of the process, before a discussion about modulating it can take place (BOX) (FIGURE 1).
Cells undergo the following changes during apoptosis:
* Membrane blebbing (bulges of the cell membrane)
* Cytoplasmic and nuclear condensation. The cell volume decreases
* Fragmentation of the DNA content
* Formation of apoptotic bodies (sealed membrane vesicles containing left-over material)
* The resulting material is then ingested by other parenchymal cells.
Figure 1. The different elements involved in apoptosis. In reality, the process is much more complicated. Image credit: www.biomedcentral.com
There are three phases during apoptosis. These are:
A. The Induction phase: This is influenced by signals which stimulate pro-apoptotic cascades. Initiating signals may be released by free radical damage, UV radiation and DNA mutations. There are several mechanisms which converge into a single event: the cell’s commitment to die, and this depends on the caspases. Caspases (Cysteine-dependent aspartate-directed proteases) are proteases which are classified into several sub groups, having a role in different stages of apoptosis.
Caspases must be pre-activated before they become effective. One way of activating the caspases are via Apafs (Apoptotic Protease Activating Factors, which are the results of an earlier DNA damage). Their activation depends on the balance between pro and anti-apoptotic factors. Once activated, the caspases activate other caspases all of which cleave cell proteins leading to apoptosis.
B. The Effector phase: In this second phase, the cell becomes committed to die. This may happen by activation of death receptors which, again, lead to the activation of pro-apoptotic caspases.
C. The Degradation Phase: When the effector phase is over, the degradation phase starts. During this phase, the activated caspases cleave cytoplasmic and nuclear material. The cell is fragmented and phagocytosis starts, particularly by neighbouring macrophages.
Apoptosis in Aging
I will mention a few representative examples of apoptosis during age-related degeneration.
In Alzheimer’s disease there is an increased activity of amyloid beta proteins, which results in accumulation of aggregates of damaged proteins. This increased amount of amyloid beta stimulates apoptosis, resulting in loss of function due to too many neurons being eliminated. Oestrogens may reduce the rate of apoptosis in the brain. This may be due to the ability of oestrogens to induce the expression of an anti-apoptotic protein Bcl-XL which restricts the activity of amyloid beta and thus, ultimately, reduces the risk of dementia.
Aging causes apoptotic loss of cardiac muscle cells. Cells may also be lost through ischaemic events which cause necrosis. Drugs which have an action on the heart may reduce apoptosis. For example, enalapril was found to reduce cardiomyocyte apoptosis in animals (2). Whilst writing, I will also mention that enalapril was found to be effective in protecting neurons against apoptosis and it may thus be useful in dementia (3).
Aging is associated with increased coagulability of the blood. Chronic inflammation reactions are associated with increased blood viscosity and blood clotting, resulting in obstruction of small capillaries and local ischaemia. This initiates necrosis is cells within the ischaemic area and apoptosis of cells around the ischaemic area. Therefore, any remedies which may modulate coagulation should also able to balance apoptosis and prevent excessive tissue dysfunction. For example, glucosamine was found to be a mild anticoagulant, ginkgo biloba and vitamin E are well known mild anticoagulants and bromelain (extract of pineapple) is thought to help reduce blood viscosity.
Modulators of Apoptosis
It is worth remembering that the regulation of apoptotic mechanisms is finely balanced. Any aberration of this balance may result in pathologies seen in aging and cancer. Excessive apoptosis causes organ atrophy, whereas slow or inappropriate apoptosis allows damaged cells to accumulate and increases the risk of these turning malignant. Autoimmune diseases are thought to be caused by reduced apoptosis. Immune cells able to react with ‘self’ are not eliminated for some reason, and stay behind causing chronic inflammation and autoimmune reactions such as those resulting in rheumatoid arthritis.
The aim of anti-aging practitioners is to reduce apoptosis in healthy older people (and thus prevent excessive cell loss and organ atrophy). The aim in cancer treatment is to increase apoptosis in cancer cell lines and thus eliminate as many cancer cells as possible (FIGURE 2).
Figure 2. Another view of the many pathways involved in apoptosis. Image credit: http://www.wikipathways.org/index.php/Pathway:WP1772
Some possible anti-apoptotic agents, that may be used in aging are:
1. Caspase modulators. Caspases-inhibitors can control apoptosis. There are positive pre-clinical reports of benefit in Parkinson’s disease and in brain injury. Promising results were also achieved with in vitro experiments (FIGURE 3).
Figure 3. The Caspase 2 selective inhibitor Z-VDVAD-FMK (Image from http://being-bioreactive.com/2015/05/27/how-to-modulate-or-inhibit-caspase-activities/
2. Nicotinamide, Acetyl L-carnitine, and aspirin are well known modulators (4). These are thought to work by protecting against oxidation, which is the first step in apoptosis modulation.
3. Idebenone is anti-apoptotic. Idebenone can prevent cytochrome c release, and inhibit MPT. This stands for ‘Mitochondrial Permeability Transition’, which increases during apoptosis. MPT-blockers therefore inhibit apoptosis. Idebenone and coenzyme Q10 are particularly active in the mitochondria.
4. Aminoguanidine is a pluripotent agent (antioxidant and antiglycator) which exhibits anti-apoptotic properties (5). Aminoguanidine inhibits advanced glycation end products (AGEs) and suppresses inflammatory markers such as NF-kB. By doing so, it inhibits the initiation of apoptosis and it has been found to protect against premature apoptotic loss of retinal cells (6).
5. Glutathione is a promising apoptotic modulator (7). This is involved in apoptotic signalling, oxidation damage and protein modulation, in the initiation of the apoptotic cascade.
6. The IAP (Inhibitor of Apoptosis) family of proteins are over-active in cancer (as they inhibit apoptosis, they allow cancerous cells to accumulate). These proteins regulate caspases and ubiquitin which, in turn, influence immunity and cell signalling. Research is aiming to find peptides which modulate IAP (8). Several peptidomimetics (small peptides which mimic the action of IAP) are being developed and tested.
Pro-apoptotics (for Cancer Treatment)
1. Recombinant TRAIL for use in lung, breast colon and kidney cancers.
TRAIL (TNF-related apoptosis-inducing ligand) is a ligand which binds to Tumor Necrosis Factor receptors inducing apoptosis. It is one of the hottest areas of research at present, and the hope is that recombinant TRAIL may be the next generation therapy in cancer. Remember however that TRAIL may not be what is needed in healthy ageing (i.e. non-cancer) because it increases the rate of apoptosis and thus may result in organ failure.
2. Indole 3 Carbinol stimulates apoptosis (and reduces cancer risk). It Induces apoptosis in human cervical cancer cell lines. It activates the caspases, and rebalances the rate of anti and pro-apoptotic proteins. This effect was also seen in prostate cancer and in several other cancers (9).
In conclusion, this is a very promising area of research that needs our attention, as there are several agents and compounds that can be used in achieving positive results. This is not to say that the currently available or future agents will be fully effective, but at least we have some information and some facts which we can work on.
1. Kerr JF, Wyllie AH, Currie AR. Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br J Cancer. 1972 Aug;26(4):239-57
2. Kushwaha S, Vikram A, Jena GB. Protective effects of enalapril in streptozotocin-induced diabetic rat: studies of DNA damage, apoptosis and expression of CCN2 in the heart, kidney and liver. J Appl Toxicol. 2012 Sep;32(9):662-72
3. Meamar R et al. Enalapril protects endothelial cells against induced apoptosis in Alzheimer’s disease. J Res Med Sci. 2013 Mar;18(Suppl 1):S1-5
4. Zhang ZY et al. Acetyl-l-carnitine ameliorate mitochondrial damage and apoptosis following spinal cord injury in rats. Neurosci Lett. 2015 Jun 12. [Epub ahead of print]
5. Orman D. et al. Aminoguanidine mitigates apoptosis, testicular seminiferous tubules damage, and oxidative stress in streptozotocin-induced diabetic rats. Tissue Cell. 2015 Jun;47(3):284-90
6. Kim J et al. Aminoguanidine protects against apoptosis of retinal ganglion cells in Zucker diabetic fatty rats. Eur Rev Med Pharmacol Sci. 2014 Jun;18(11):1573-8
7. Magdalena L et al. Glutathione and modulation of cell apoptosis Biochimica et Biophysica Acta (BBA) – Molecular Cell Research, 2012. Vol.1823 (10)1767–1777
8. Silke J, Meier P. Inhibitor of Apoptosis (IAP) Proteins–Modulators of Cell Death and Inflammation. doi: 10.1101/cshperspect.a008730, 2013
9. Safa M. et al. Indole-3-carbinol suppresses NF-κB activity and stimulates the p53 pathway in pre-B acute lymphoblastic leukemia cells. Tumour Biol. 2015 May;36(5):3919-30